Vertical Profiles of Aeolian Dust in a Desert Atmosphere Observed using Lidar in Shapotou, China

We analyzed the time series data of dust vertical profiles, observed using polarization elastic lidar, from April 8 t h -21 s t , 2002, during the first intensive observation period of the Aeolian Dust Experiment on Climateimpact (ADEC), in Shapotou, Ninxia Province in China. We conducted the analysis with consideration to the upper atmospheric soundings and surface weather data. The results are summarized as follows. 1) A well-mixed dust layer dominated in the boundary layer sampled from around 11 a.m. until around 1 or 2 a.m. local time (Beijing time). Under well-mixed conditions, the mixed layers grew well with dry convections in the boundary layer, especially during the day and in the early evening. 2) Dry convections developed deeper during synoptic low-pressure conditions because of the systematic upward motions prevalent in the low-pressure systems. Vertical wind shear affected growth of the mixed layer as well, that is, weak vertical wind shear condition was favorable for the growth of the mixed layer. The deepest mixed layer was observed on April 13 t h at a thickness of about 6-km above ground, when the pressure was the lowest, the vertical wind shear was very weak, and the atmosphere was dry. The aspect ratios (horizontal/vertical scale ratios) of the convections decreased under low-pressure and weak-shear conditions. On the other hand, the aspect ratios of the convections increased dramatically under high-pressure and strong-shear conditions, because convection growth was suppressed by systematic down-drafts of the high-pressure system and by strong vertical wind shears.